The present invention relates in general to an improved data communication and networking system, and in particular, to an improved system and method for conveying data between a transparent bridging domain and a source route bridging domain.
The prior art provides two bridging architectures that can be used to interconnect local area networks (LANs). Both the source route bridging (SRB) architecture and the transparent bridging (TB) architecture are well-known in the prior art and will only be described briefly here to describe the environment in which the invention is used.
In the source route bridging architecture, the specific route which a frame must traverse between a source end station (host) and a destination end station (host) is carried in the frame. SRBs are so-named because the complete source-to-destination route is placed in all inter-LAN frames sent by the source station. SRBs store and forward the frames as indicated by the route appearing in the appropriate field of the frame. Before the frame is transmitted with routing information, the source station sends an explorer frame which is propagated throughout the network to the destination station. As the explorer frame traverses the network, a source route bridge interconnecting LANs enters routing information in the frame such as bridge identification number, LAN number, etc. When the explorer frame arrives at the destination end station, it is copied and then rerouted by the specific route entered into the frame to the source end station. The source end station stores the route information and uses it to send subsequent frames to the destination end station. A more detailed description of the source routing architecture is contained in the IEEE 802.5 token ring LAN specification.
In the transparent bridging architecture, routing information is not carried in the frame. Transparent bridges are so-named because their presence and operation is transparent to network hosts. When transparent bridges are powered on, they learn the topology of the network by analyzing the source address of incoming frames from all attached LANs. When a transparent bridge receives a frame from a host on an interface (port), it concludes that the host can be reached through the LAN connected to that port. The transparent bridge uses this process to build a table containing the host address and the interface number. When a source station on one LAN intends to communicate with a destination end station on another LAN, the transparent bridge compares the destination address with the entries in the transparent bridge table. If the table contains an association between the destination address and any of the bridge""s ports except the one on which the frame was received, the frame is forwarded out the indicated port. If no association is found, the frame is flooded to all ports except the port on which it was received. Broadcast frames and multicast frames are flooded in the same way. A more detailed description of the transparent bridging architecture is contained in the IEEE 802.1 standard.
Transparent bridges are found predominantly in Ethernet IEEE 802.3 networks and source route bridges are found almost exclusively in token ring networks. Translational bridges are designed to bridge between a transparent bridging domain and a source route bridging domain in a bridged heterogeneous communications network. Translational bridging has not been standardized by organizations such as the IEEE, with the result that many aspects of translational bridging have been left to the product vendor implementing this function. IBM introduced source route transparent (SRT) bridging as an alternative to translational bridging in a heterogeneous bridging environment. SRT bridges can forward traffic from both transparent and source route end stations and form a common spanning tree with transparent bridges, allowing end stations of each type to communicate with end stations of the same type in a network of arbitrary topology. U.S. Pat. No. 5,280,480 teaches source route transparent bridging and is hereby incorporated by reference.
Ultimately, the goal of connecting a transparent bridging domain and a source route bridging domain is to allow communication between hosts in the transparent bridging domain and hosts in the source route bridging domain. Ethernet and token ring LANs both support 48 bit media access control (MAC) addresses, although the internal hardware representation of these addresses differ. Although MAC addresses are unique for Ethernet devices, the token ring protocol allows duplicate MAC addresses, that is, stations on different token rings can have the same token ring MAC address. The problem specifically addressed by the invention is that of distributing traffic originating at different hosts in the transparent bridging domain along different routes to a destination address in the source route bridging domain when there are multiple instances of the destination address.
It is therefore one object of this invention to provide an improved data communication and networking system in a mixed bridging environment.
It is another object of this invention to provide a method for distributing traffic originating from hosts in a transparent bridging environment along multiple communications paths to a destination MAC address.
It is another object of the invention to provide an improved translational bridge that supports multiple active communication paths for multiple instances of the destination MAC address in the source route bridging network.
The foregoing objects are achieved by the method and system of the present invention which provides a mechanism that allows translational bridges connecting transparently bridged networks to source route bridged networks to support multiple communication paths for multiple instances of the MAC address in the SRB network.
Specifically, the invention allows traffic from transparently bridged clients destined to a MAC address on the SRB network on the other side of the translational bridge to be distributed across multiple instances of the same MAC address. The translational bridge sends a spanning tree explorer (STE) frame if the destination MAC address has not been learned. This ensures that the parallel paths to the destination MAC address are discovered. The STE frame is delivered once to each ring in the SRB network. The translational bridge receives a response from the destination MAC over each ring with corresponding routing information fields (RIFs) which are then cached. The translational bridge then distributes traffic destined to this MAC address across the different RIFs. Load balancing is performed by using the MAC address of the source as the key, i.e., frames from each transparently bridged client to the same destination MAC address can be distributed over separate RIFs.